1. Field of the Invention
The present invention relates to a ferroelectric thin-film element having a buffer layer with a specified crystal orientation formed on a single-crystal semiconductor substrate. More particularly, the present invention relates to an oriented ferroelectric thin-film element that permits a nonvolatile memory, a capacitor or an optical modulator to be fabricated on a semiconductor substrate.
2. Description of the Related Art
Because of the versatile properties possessed by ferroelectrics such as ferroelectricity, piezoelectricity, pyroelectricity and electrooptic effects, oxide ferroelectric thin films have so far been held to have potential use in nonvolatile memories and in many other applications such as surface acoustic wave devices, infrared pyroelectric devices, optoacoustic devices and electrooptic devices. In some of these applications, it is necessary to reduce the optical loss that occurs in thin-film optical waveguide structures and it is also required to insure polarizing characteristics and electrooptic effects comparable to those of single crystals. To meet these requirements, the preparation of single-crystal thin films is essential. To this end, many attempts have been made to form epitaxial ferroelectric thin films of BaTiO.sub.3, PbTiO.sub.3, Pb.sub.1-x La.sub.x (Zr.sub.1-y Ti.sub.y).sub.i-x/4 O.sub.3 (PLZT), LiNbO.sub.3, KNbO.sub.3, Bi.sub.4 Ti.sub.3 O.sub.12, etc. on oxide single-crystal substrates by suitable techniques such as Rf-magnetron sputtering, ion-beam sputtering, laser ablation and metal organic chemical vapor deposition (MOCVD).
However, to achieve integration with semiconductor devices, the ferroelectric thin film must be formed on a semiconductor substrate.
For the formation of ferroelectric thin films on semiconductor substrates, it has been proposed that buffer layers capable of epitaxial growth at low temperatures be provided on semiconductor substrates. For example, a buffer layer of MgAl.sub.2 O.sub.4 (100) or MgO (100) is allowed to grow spitaxially on single-crystal Si (100) as a substrate and a ferroelectric compound is then allowed to grow epitaxially on the substrate, as disclosed in Unexamined Japanese Patent Publication Sho. 61-185808. However, this publication fails to teach or suggest the crystallographic relationship between Si (100) and MgO (100). A later study has shown that when (100) oriented MgO is formed on single-crystal Si (100), the MgO simply has a (100) face parallel to a (100) face in Si but its in-plane directions are random; in other words, it is oriented polycrystalline MgO [P. Tiwari et al.; J. Appl. Phys., 69, 8358 (1991)].
The present inventors previously proposed a method in which a MgO (100) buffer layer was allowed to grow epitaxially on a GaAs semiconductor (100) substrate and then overlaid with an epitaxial or oriented ferroelectric thin film in Japanese Patent Application No. Hei. 4-319229. In this case, the following crystallographic relationships hold as regards BaTiO.sub.3 on GaAs: BaTiO.sub.3 (001)//MgO(100)//GaAs(100); in-plane directions BaTiO.sub.3 [010]//Mg[001]//GaAs[001].
Particularly, Ga-As materials are used in the active layer of a semiconductor laser, and which have to be (100) orientated crystal so as to be the material for the active layer of the semiconductor. A single crystal material such as Lithium niobate used for the optical waveguide have to be orientated as a (0001) single crystal and is integrated with the semiconductor laser using Ga-As materials into one assembly so as to resonate on the surface of the optical waveguide. However, it is impossible due to the difference of the orientation of their crystal surface.
The previous proposal, however, has had the following problem. The lattice constants of the ferroelectric thin film and the symmetry of the crystal lattice are close to those of a (100) face in the MgO buffer layer. Therefore, except in tetrahedral ferroelectrics having the axes of polarization in a [001] direction, it has been impossible to align the axes of polarization in one direction over the semiconductor (100) substrate, For example, the lattice constants and symmetry of the crystal lattice of an orthorhombic ferroelectric having the axes of polarization in a [111] direction and a hexagonal ferroelectric having the axes of polarization in a [0001] direction differ from those of a (100) face in MgO. Accordingly, an orthorhombic ferroelectric thin film having crystal orientation in a (111) face or a hexagonal ferroelectric thin film having (0001) orientation cannot be formed on the semiconductor (100) substrate.